sysutils/screen: Ignore legacy pty (pty.ko) during build

screen built on a machine with pty.ko (legacy SYSVR4 pty support)
loaded will fail to run on machines without pty.ko loaded. It will
try to open /dev/ptmx and fail, crashing screen. This commit fixes
this by removing the test for /dev/ptmx (legacy SYSVR4 pty support).

Reported by: Jonathan Chen <jonc@chen.org.nz>